Recently, inexpensive torsional hinged flat mirrors with a single reflective surface have gained acceptance as a reliable replacement scanning mirror for the much more expensive rotating polygon mirrors used in laser printers. Laser printers use a scanning mirror to provide a continuous sweep or scan of a modulated light source across a photosensitive medium such as a rotating drum. By designing the torsional hinged mirror to have a resonant frequency substantially at the desired scanning or sweep frequency of the laser beam, these torsional hinged mirrors are used in new generations of high-speed laser printers at a very advantageous cost.
Texas Instruments presently manufactures a large number of mirror MEMS devices fabricated or etched from a single piece of material (such as a silicon wafer for example) using semiconductor manufacturing processes. Before the mirror devices are etched, at least one surface (and usually both) of the silicon wafer is polished to provide the reflective surface of the mirror, which may have any suitable perimeter shape such as oval elongated elliptical, rectangular, square or other. Single axis mirrors include the reflective surface portion and a pair of torsional or full hinges, which extend to a support frame or alternately the hinges may extend from the mirror portion to a pair of hinge anchors. Other mirror embodiments use a single torsional hinge to eliminate hinge stress resulting from temperature changes and mounting stresses.
Although several types of drive techniques may be used to drive (i.e., initiate and maintain resonant oscillation of the mirror), various magnetic drives have been found to be especially suitable.
Generally, a magnetic drive suitable for use with such torsional hinged mirrors comprise one or more permanent magnets bonded or mounted on the pivoting structure portion of the mirror. One or more electromagnetic coil(s) is separated from, but is positioned proximate the permanent magnet. According to the prior art, a bipolar sine wave or electrical signal having a frequency substantially at the resonant frequency of the torsional hinged mirror device was applied across the electromagnetic coil(s) to initiate and maintain pivotal oscillation of the torsional hinged mirror at the resonant frequency.
Although the torsional hinged resonant mirror bipolar apparatus is inexpensive compared to rotating polygon mirrors, the sine wave type power supply having a frequency that can be calibrated to the resonant frequency of the mirror constitutes one of the more expensive items. Therefore, it would be still more advantageous to use an inexpensive and effective power supply and technique to drive the resonant torsional hinged device.